Manning's treatment is "emblematic of a major phenomenon occurring right now," said Dr. Joshua Hare, director of the Interdisciplinary Stem Cell Institute at the University of Miami Miller School of Medicine, who is not involved with Manning's therapy.

The treatment used by Manning, the sidelined quarterback for the Indianapolis Colts, is currently not approved in the United States, according to Fox. Going abroad for such a therapy is a practice being called "medical tourism."

Traveling far and wide

In the United States, new therapies are subject to very rigorous testing to try to ensure they work and will not cause harm, Hare said. Currently, there is not enough evidence to say whether many stem cell therapies, such as the one Manning received, are effective, he said.

Consequently, Hare said he doesn't know if Manning's treatment will do any good. "I'm excited about the possibility that [it] might work, but I'm a big advocate for doing clinical trials," Hare said.

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The patients increasingly resorting to medical tourism suffer from a variety of conditions, including heart disease, stroke, Parkinson's disease and autism, Hare said.

"It just speaks to the sense that patients believe that stem cell treatment can do something for them that regular medicine can't," Hare said. This may be because patients intuitively think cells from their own body are effective treatments, or because they have exhausted more conventional treatments, he said.

Whatever the reason, "there's a real need for us [in the United States] to hurry up with some of the clinical research," Hare said.

Assuming news reports are accurate, Manning's treatment may have used what are called mesenchymal cells, Hare said.

These are adult stem cells found in fat, bone marrow and other tissues, Hare said. Unlike embryonic stem cells, which can develop into any cell in the body, adult stem cells have a more limited fate.

Mesenchymal cells can develop into bone, cartilage, fat or muscle. "So they could, in theory … then be used to treat diseases of bone, cartilage, tendon and muscle," said Hare, who is currently researching whether these cells can be used to repair heart tissue.

A bad example

"We do need to find out whether these treatments are safe and how to optimize them. The medical tourism makes it even more urgent," Hare added.

Others are concerned Manning's use of stem cell therapy may set a bad example and lead others to seek out the same, unproven treatment.

Larry Goldstein, director of the University of California, San Diego, Stem Cell Program, said in an email: "When a highly visible celebrity athlete chooses to undergo an untested/unproven therapy, and if they happen to get better without knowing whether the therapy is what caused the improvement, they encourage many other people to ignore scientific evidence and to substitute hope and blind trust for proof."

"The downside is that many people might be hurt by subjecting themselves to a risky procedure, or procedure with unknown risks, when there is no evidence of benefit to be gained."

Goldstein added: "I would hope that he [Manning] would publicly address the absence of evidence about this proposed procedure."

Pass it on: Peyton Manning's reported stem cell therapy has not been proven to work.